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<br/>
The Library<br/>
-----------<br/>
<br/>
This paper is covers two major areas:<br/>
<br/>
- Features and policies not mentioned in the standard that<br/>
the quality of the library implementation depends on, including<br/>
extensions and "implementation-defined" features;<br/>
<br/>
- Plans for required but unimplemented library features and<br/>
optimizations to them.<br/>
<br/>
Overhead<br/>
--------<br/>
<br/>
The standard defines a large library, much larger than the standard<br/>
C library. A naive implementation would suffer substantial overhead<br/>
in compile time, executable size, and speed, rendering it unusable<br/>
in many (particularly embedded) applications. The alternative demands<br/>
care in construction, and some compiler support, but there is no<br/>
need for library subsets.<br/>
<br/>
What are the sources of this overhead? There are four main causes:<br/>
<br/>
- The library is specified almost entirely as templates, which<br/>
with current compilers must be included in-line, resulting in<br/>
very slow builds as tens or hundreds of thousands of lines<br/>
of function definitions are read for each user source file.<br/>
Indeed, the entire SGI STL, as well as the dos Reis valarray,<br/>
are provided purely as header files, largely for simplicity in<br/>
porting. Iostream/locale is (or will be) as large again.<br/>
<br/>
- The library is very flexible, specifying a multitude of hooks<br/>
where users can insert their own code in place of defaults.<br/>
When these hooks are not used, any time and code expended to<br/>
support that flexibility is wasted.<br/>
<br/>
- Templates are often described as causing to "code bloat". In<br/>
practice, this refers (when it refers to anything real) to several<br/>
independent processes. First, when a class template is manually<br/>
instantiated in its entirely, current compilers place the definitions<br/>
for all members in a single object file, so that a program linking<br/>
to one member gets definitions of all. Second, template functions<br/>
which do not actually depend on the template argument are, under<br/>
current compilers, generated anew for each instantiation, rather<br/>
than being shared with other instantiations. Third, some of the<br/>
flexibility mentioned above comes from virtual functions (both in<br/>
regular classes and template classes) which current linkers add<br/>
to the executable file even when they manifestly cannot be called.<br/>
<br/>
- The library is specified to use a language feature, exceptions,<br/>
which in the current gcc compiler ABI imposes a run time and<br/>
code space cost to handle the possibility of exceptions even when<br/>
they are not used. Under the new ABI (accessed with -fnew-abi),<br/>
there is a space overhead and a small reduction in code efficiency<br/>
resulting from lost optimization opportunities associated with<br/>
non-local branches associated with exceptions.<br/>
<br/>
What can be done to eliminate this overhead? A variety of coding<br/>
techniques, and compiler, linker and library improvements and<br/>
extensions may be used, as covered below. Most are not difficult,<br/>
and some are already implemented in varying degrees.<br/>
<br/>
Overhead: Compilation Time<br/>
--------------------------<br/>
<br/>
Providing "ready-instantiated" template code in object code archives<br/>
allows us to avoid generating and optimizing template instantiations<br/>
in each compilation unit which uses them. However, the number of such<br/>
instantiations that are useful to provide is limited, and anyway this<br/>
is not enough, by itself, to minimize compilation time. In particular,<br/>
it does not reduce time spent parsing conforming headers.<br/>
<br/>
Quicker header parsing will depend on library extensions and compiler<br/>
improvements. One approach is some variation on the techniques<br/>
previously marketed as "pre-compiled headers", now standardized as<br/>
support for the "export" keyword. "Exported" template definitions<br/>
can be placed (once) in a "repository" -- really just a library, but<br/>
of template definitions rather than object code -- to be drawn upon<br/>
at link time when an instantiation is needed, rather than placed in<br/>
header files to be parsed along with every compilation unit.<br/>
<br/>
Until "export" is implemented we can put some of the lengthy template<br/>
definitions in #if guards or alternative headers so that users can skip<br/>
over the full definitions when they need only the ready-instantiated<br/>
specializations.<br/>
<br/>
To be precise, this means that certain headers which define<br/>
templates which users normally use only for certain arguments<br/>
can be instrumented to avoid exposing the template definitions<br/>
to the compiler unless a macro is defined. For example, in<br/>
<string>, we might have:<br/>
<br/>
template <class _CharT, ... > class basic_string {<br/>
... // member declarations<br/>
};<br/>
... // operator declarations<br/>
<br/>
#ifdef _STRICT_ISO_<br/>
# if _G_NO_TEMPLATE_EXPORT<br/>
# include <bits/std_locale.h> // headers needed by definitions<br/>
# ...<br/>
# include <bits/string.tcc> // member and global template definitions.<br/>
# endif<br/>
#endif<br/>
<br/>
Users who compile without specifying a strict-ISO-conforming flag<br/>
would not see many of the template definitions they now see, and rely<br/>
instead on ready-instantiated specializations in the library. This<br/>
technique would be useful for the following substantial components:<br/>
string, locale/iostreams, valarray. It would *not* be useful or<br/>
usable with the following: containers, algorithms, iterators,<br/>
allocator. Since these constitute a large (though decreasing)<br/>
fraction of the library, the benefit the technique offers is<br/>
limited.<br/>
<br/>
The language specifies the semantics of the "export" keyword, but<br/>
the gcc compiler does not yet support it. When it does, problems<br/>
with large template inclusions can largely disappear, given some<br/>
minor library reorganization, along with the need for the apparatus<br/>
described above.<br/>
<br/>
Overhead: Flexibility Cost<br/>
--------------------------<br/>
<br/>
The library offers many places where users can specify operations<br/>
to be performed by the library in place of defaults. Sometimes<br/>
this seems to require that the library use a more-roundabout, and<br/>
possibly slower, way to accomplish the default requirements than<br/>
would be used otherwise.<br/>
<br/>
The primary protection against this overhead is thorough compiler<br/>
optimization, to crush out layers of inline function interfaces.<br/>
Kuck & Associates has demonstrated the practicality of this kind<br/>
of optimization.<br/>
<br/>
The second line of defense against this overhead is explicit<br/>
specialization. By defining helper function templates, and writing<br/>
specialized code for the default case, overhead can be eliminated<br/>
for that case without sacrificing flexibility. This takes full<br/>
advantage of any ability of the optimizer to crush out degenerate<br/>
code.<br/>
<br/>
The library specifies many virtual functions which current linkers<br/>
load even when they cannot be called. Some minor improvements to the<br/>
compiler and to ld would eliminate any such overhead by simply<br/>
omitting virtual functions that the complete program does not call.<br/>
A prototype of this work has already been done. For targets where<br/>
GNU ld is not used, a "pre-linker" could do the same job.<br/>
<br/>
The main areas in the standard interface where user flexibility<br/>
can result in overhead are:<br/>
<br/>
- Allocators: Containers are specified to use user-definable<br/>
allocator types and objects, making tuning for the container<br/>
characteristics tricky.<br/>
<br/>
- Locales: the standard specifies locale objects used to implement<br/>
iostream operations, involving many virtual functions which use<br/>
streambuf iterators.<br/>
<br/>
- Algorithms and containers: these may be instantiated on any type,<br/>
frequently duplicating code for identical operations.<br/>
<br/>
- Iostreams and strings: users are permitted to use these on their<br/>
own types, and specify the operations the stream must use on these<br/>
types.<br/>
<br/>
Note that these sources of overhead are _avoidable_. The techniques<br/>
to avoid them are covered below.<br/>
<br/>
Code Bloat<br/>
----------<br/>
<br/>
In the SGI STL, and in some other headers, many of the templates<br/>
are defined "inline" -- either explicitly or by their placement<br/>
in class definitions -- which should not be inline. This is a<br/>
source of code bloat. Matt had remarked that he was relying on<br/>
the compiler to recognize what was too big to benefit from inlining,<br/>
and generate it out-of-line automatically. However, this also can<br/>
result in code bloat except where the linker can eliminate the extra<br/>
copies.<br/>
<br/>
Fixing these cases will require an audit of all inline functions<br/>
defined in the library to determine which merit inlining, and moving<br/>
the rest out of line. This is an issue mainly in chapters 23, 25, and<br/>
27. Of course it can be done incrementally, and we should generally<br/>
accept patches that move large functions out of line and into ".tcc"<br/>
files, which can later be pulled into a repository. Compiler/linker<br/>
improvements to recognize very large inline functions and move them<br/>
out-of-line, but shared among compilation units, could make this<br/>
work unnecessary.<br/>
<br/>
Pre-instantiating template specializations currently produces large<br/>
amounts of dead code which bloats statically linked programs. The<br/>
current state of the static library, libstdc++.a, is intolerable on<br/>
this account, and will fuel further confused speculation about a need<br/>
for a library "subset". A compiler improvement that treats each<br/>
instantiated function as a separate object file, for linking purposes,<br/>
would be one solution to this problem. An alternative would be to<br/>
split up the manual instantiation files into dozens upon dozens of<br/>
little files, each compiled separately, but an abortive attempt at<br/>
this was done for <string> and, though it is far from complete, it<br/>
is already a nuisance. A better interim solution (just until we have<br/>
"export") is badly needed.<br/>
<br/>
When building a shared library, the current compiler/linker cannot<br/>
automatically generate the instantiations needed. This creates a<br/>
miserable situation; it means any time something is changed in the<br/>
library, before a shared library can be built someone must manually<br/>
copy the declarations of all templates that are needed by other parts<br/>
of the library to an "instantiation" file, and add it to the build<br/>
system to be compiled and linked to the library. This process is<br/>
readily automated, and should be automated as soon as possible.<br/>
Users building their own shared libraries experience identical<br/>
frustrations.<br/>
<br/>
Sharing common aspects of template definitions among instantiations<br/>
can radically reduce code bloat. The compiler could help a great<br/>
deal here by recognizing when a function depends on nothing about<br/>
a template parameter, or only on its size, and giving the resulting<br/>
function a link-name "equate" that allows it to be shared with other<br/>
instantiations. Implementation code could take advantage of the<br/>
capability by factoring out code that does not depend on the template<br/>
argument into separate functions to be merged by the compiler.<br/>
<br/>
Until such a compiler optimization is implemented, much can be done<br/>
manually (if tediously) in this direction. One such optimization is<br/>
to derive class templates from non-template classes, and move as much<br/>
implementation as possible into the base class. Another is to partial-<br/>
specialize certain common instantiations, such as vector<T*>, to share<br/>
code for instantiations on all types T. While these techniques work,<br/>
they are far from the complete solution that a compiler improvement<br/>
would afford.<br/>
<br/>
Overhead: Expensive Language Features<br/>
-------------------------------------<br/>
<br/>
The main "expensive" language feature used in the standard library<br/>
is exception support, which requires compiling in cleanup code with<br/>
static table data to locate it, and linking in library code to use<br/>
the table. For small embedded programs the amount of such library<br/>
code and table data is assumed by some to be excessive. Under the<br/>
"new" ABI this perception is generally exaggerated, although in some<br/>
cases it may actually be excessive.<br/>
<br/>
To implement a library which does not use exceptions directly is<br/>
not difficult given minor compiler support (to "turn off" exceptions<br/>
and ignore exception constructs), and results in no great library<br/>
maintenance difficulties. To be precise, given "-fno-exceptions",<br/>
the compiler should treat "try" blocks as ordinary blocks, and<br/>
"catch" blocks as dead code to ignore or eliminate. Compiler<br/>
support is not strictly necessary, except in the case of "function<br/>
try blocks"; otherwise the following macros almost suffice:<br/>
<br/>
#define throw(X)<br/>
#define try if (true)<br/>
#define catch(X) else if (false)<br/>
<br/>
However, there may be a need to use function try blocks in the<br/>
library implementation, and use of macros in this way can make<br/>
correct diagnostics impossible. Furthermore, use of this scheme<br/>
would require the library to call a function to re-throw exceptions<br/>
from a try block. Implementing the above semantics in the compiler<br/>
is preferable.<br/>
<br/>
Given the support above (however implemented) it only remains to<br/>
replace code that "throws" with a call to a well-documented "handler"<br/>
function in a separate compilation unit which may be replaced by<br/>
the user. The main source of exceptions that would be difficult<br/>
for users to avoid is memory allocation failures, but users can<br/>
define their own memory allocation primitives that never throw.<br/>
Otherwise, the complete list of such handlers, and which library<br/>
functions may call them, would be needed for users to be able to<br/>
implement the necessary substitutes. (Fortunately, they have the<br/>
source code.)<br/>
<br/>
Opportunities<br/>
-------------<br/>
<br/>
The template capabilities of C++ offer enormous opportunities for<br/>
optimizing common library operations, well beyond what would be<br/>
considered "eliminating overhead". In particular, many operations<br/>
done in Glibc with macros that depend on proprietary language<br/>
extensions can be implemented in pristine Standard C++. For example,<br/>
the chapter 25 algorithms, and even C library functions such as strchr,<br/>
can be specialized for the case of static arrays of known (small) size.<br/>
<br/>
Detailed optimization opportunities are identified below where<br/>
the component where they would appear is discussed. Of course new<br/>
opportunities will be identified during implementation.<br/>
<br/>
Unimplemented Required Library Features<br/>
---------------------------------------<br/>
<br/>
The standard specifies hundreds of components, grouped broadly by<br/>
chapter. These are listed in excruciating detail in the CHECKLIST<br/>
file.<br/>
<br/>
17 general<br/>
18 support<br/>
19 diagnostics<br/>
20 utilities<br/>
21 string<br/>
22 locale<br/>
23 containers<br/>
24 iterators<br/>
25 algorithms<br/>
26 numerics<br/>
27 iostreams<br/>
Annex D backward compatibility<br/>
<br/>
Anyone participating in implementation of the library should obtain<br/>
a copy of the standard, ISO 14882. People in the U.S. can obtain an<br/>
electronic copy for US$18 from ANSI's web site. Those from other<br/>
countries should visit http://www.iso.org/ to find out the location<br/>
of their country's representation in ISO, in order to know who can<br/>
sell them a copy.<br/>
<br/>
The emphasis in the following sections is on unimplemented features<br/>
and optimization opportunities.<br/>
<br/>
Chapter 17 General<br/>
-------------------<br/>
<br/>
Chapter 17 concerns overall library requirements.<br/>
<br/>
The standard doesn't mention threads. A multi-thread (MT) extension<br/>
primarily affects operators new and delete (18), allocator (20),<br/>
string (21), locale (22), and iostreams (27). The common underlying<br/>
support needed for this is discussed under chapter 20.<br/>
<br/>
The standard requirements on names from the C headers create a<br/>
lot of work, mostly done. Names in the C headers must be visible<br/>
in the std:: and sometimes the global namespace; the names in the<br/>
two scopes must refer to the same object. More stringent is that<br/>
Koenig lookup implies that any types specified as defined in std::<br/>
really are defined in std::. Names optionally implemented as<br/>
macros in C cannot be macros in C++. (An overview may be read at<br/>
<http://www.cantrip.org/cheaders.html>). The scripts "inclosure"<br/>
and "mkcshadow", and the directories shadow/ and cshadow/, are the<br/>
beginning of an effort to conform in this area.<br/>
<br/>
A correct conforming definition of C header names based on underlying<br/>
C library headers, and practical linking of conforming namespaced<br/>
customer code with third-party C libraries depends ultimately on<br/>
an ABI change, allowing namespaced C type names to be mangled into<br/>
type names as if they were global, somewhat as C function names in a<br/>
namespace, or C++ global variable names, are left unmangled. Perhaps<br/>
another "extern" mode, such as 'extern "C-global"' would be an<br/>
appropriate place for such type definitions. Such a type would<br/>
affect mangling as follows:<br/>
<br/>
namespace A {<br/>
struct X {};<br/>
extern "C-global" { // or maybe just 'extern "C"'<br/>
struct Y {};<br/>
};<br/>
}<br/>
void f(A::X*); // mangles to f__FPQ21A1X<br/>
void f(A::Y*); // mangles to f__FP1Y<br/>
<br/>
(It may be that this is really the appropriate semantics for regular<br/>
'extern "C"', and 'extern "C-global"', as an extension, would not be<br/>
necessary.) This would allow functions declared in non-standard C headers<br/>
(and thus fixable by neither us nor users) to link properly with functions<br/>
declared using C types defined in properly-namespaced headers. The<br/>
problem this solves is that C headers (which C++ programmers do persist<br/>
in using) frequently forward-declare C struct tags without including<br/>
the header where the type is defined, as in<br/>
<br/>
struct tm;<br/>
void munge(tm*);<br/>
<br/>
Without some compiler accommodation, munge cannot be called by correct<br/>
C++ code using a pointer to a correctly-scoped tm* value.<br/>
<br/>
The current C headers use the preprocessor extension "#include_next",<br/>
which the compiler complains about when run "-pedantic".<br/>
(Incidentally, it appears that "-fpedantic" is currently ignored,<br/>
probably a bug.) The solution in the C compiler is to use<br/>
"-isystem" rather than "-I", but unfortunately in g++ this seems<br/>
also to wrap the whole header in an 'extern "C"' block, so it's<br/>
unusable for C++ headers. The correct solution appears to be to<br/>
allow the various special include-directory options, if not given<br/>
an argument, to affect subsequent include-directory options additively,<br/>
so that if one said<br/>
<br/>
-pedantic -iprefix $(prefix) \<br/>
-idirafter -ino-pedantic -ino-extern-c -iwithprefix -I g++-v3 \<br/>
-iwithprefix -I g++-v3/ext<br/>
<br/>
the compiler would search $(prefix)/g++-v3 and not report<br/>
pedantic warnings for files found there, but treat files in<br/>
$(prefix)/g++-v3/ext pedantically. (The undocumented semantics<br/>
of "-isystem" in g++ stink. Can they be rescinded? If not it<br/>
must be replaced with something more rationally behaved.)<br/>
<br/>
All the C headers need the treatment above; in the standard these<br/>
headers are mentioned in various chapters. Below, I have only<br/>
mentioned those that present interesting implementation issues.<br/>
<br/>
The components identified as "mostly complete", below, have not been<br/>
audited for conformance. In many cases where the library passes<br/>
conformance tests we have non-conforming extensions that must be<br/>
wrapped in #if guards for "pedantic" use, and in some cases renamed<br/>
in a conforming way for continued use in the implementation regardless<br/>
of conformance flags.<br/>
<br/>
The STL portion of the library still depends on a header<br/>
stl/bits/stl_config.h full of #ifdef clauses. This apparatus<br/>
should be replaced with autoconf/automake machinery.<br/>
<br/>
The SGI STL defines a type_traits<> template, specialized for<br/>
many types in their code including the built-in numeric and<br/>
pointer types and some library types, to direct optimizations of<br/>
standard functions. The SGI compiler has been extended to generate<br/>
specializations of this template automatically for user types,<br/>
so that use of STL templates on user types can take advantage of<br/>
these optimizations. Specializations for other, non-STL, types<br/>
would make more optimizations possible, but extending the gcc<br/>
compiler in the same way would be much better. Probably the next<br/>
round of standardization will ratify this, but probably with<br/>
changes, so it probably should be renamed to place it in the<br/>
implementation namespace.<br/>
<br/>
The SGI STL also defines a large number of extensions visible in<br/>
standard headers. (Other extensions that appear in separate headers<br/>
have been sequestered in subdirectories ext/ and backward/.) All<br/>
these extensions should be moved to other headers where possible,<br/>
and in any case wrapped in a namespace (not std!), and (where kept<br/>
in a standard header) girded about with macro guards. Some cannot be<br/>
moved out of standard headers because they are used to implement<br/>
standard features. The canonical method for accommodating these<br/>
is to use a protected name, aliased in macro guards to a user-space<br/>
name. Unfortunately C++ offers no satisfactory template typedef<br/>
mechanism, so very ad-hoc and unsatisfactory aliasing must be used<br/>
instead.<br/>
<br/>
Implementation of a template typedef mechanism should have the highest<br/>
priority among possible extensions, on the same level as implementation<br/>
of the template "export" feature.<br/>
<br/>
Chapter 18 Language support<br/>
----------------------------<br/>
<br/>
Headers: <limits> <new> <typeinfo> <exception><br/>
C headers: <cstddef> <climits> <cfloat> <cstdarg> <csetjmp><br/>
<ctime> <csignal> <cstdlib> (also 21, 25, 26)<br/>
<br/>
This defines the built-in exceptions, rtti, numeric_limits<>,<br/>
operator new and delete. Much of this is provided by the<br/>
compiler in its static runtime library.<br/>
<br/>
Work to do includes defining numeric_limits<> specializations in<br/>
separate files for all target architectures. Values for integer types<br/>
except for bool and wchar_t are readily obtained from the C header<br/>
<limits.h>, but values for the remaining numeric types (bool, wchar_t,<br/>
float, double, long double) must be entered manually. This is<br/>
largely dog work except for those members whose values are not<br/>
easily deduced from available documentation. Also, this involves<br/>
some work in target configuration to identify the correct choice of<br/>
file to build against and to install.<br/>
<br/>
The definitions of the various operators new and delete must be<br/>
made thread-safe, which depends on a portable exclusion mechanism,<br/>
discussed under chapter 20. Of course there is always plenty of<br/>
room for improvements to the speed of operators new and delete.<br/>
<br/>
<cstdarg>, in Glibc, defines some macros that gcc does not allow to<br/>
be wrapped into an inline function. Probably this header will demand<br/>
attention whenever a new target is chosen. The functions atexit(),<br/>
exit(), and abort() in cstdlib have different semantics in C++, so<br/>
must be re-implemented for C++.<br/>
<br/>
Chapter 19 Diagnostics<br/>
-----------------------<br/>
<br/>
Headers: <stdexcept><br/>
C headers: <cassert> <cerrno><br/>
<br/>
This defines the standard exception objects, which are "mostly complete".<br/>
Cygnus has a version, and now SGI provides a slightly different one.<br/>
It makes little difference which we use.<br/>
<br/>
The C global name "errno", which C allows to be a variable or a macro,<br/>
is required in C++ to be a macro. For MT it must typically result in<br/>
a function call.<br/>
<br/>
Chapter 20 Utilities<br/>
---------------------<br/>
Headers: <utility> <functional> <memory><br/>
C header: <ctime> (also in 18)<br/>
<br/>
SGI STL provides "mostly complete" versions of all the components<br/>
defined in this chapter. However, the auto_ptr<> implementation<br/>
is known to be wrong. Furthermore, the standard definition of it<br/>
is known to be unimplementable as written. A minor change to the<br/>
standard would fix it, and auto_ptr<> should be adjusted to match.<br/>
<br/>
Multi-threading affects the allocator implementation, and there must<br/>
be configuration/installation choices for different users' MT<br/>
requirements. Anyway, users will want to tune allocator options<br/>
to support different target conditions, MT or no.<br/>
<br/>
The primitives used for MT implementation should be exposed, as an<br/>
extension, for users' own work. We need cross-CPU "mutex" support,<br/>
multi-processor shared-memory atomic integer operations, and single-<br/>
processor uninterruptible integer operations, and all three configurable<br/>
to be stubbed out for non-MT use, or to use an appropriately-loaded<br/>
dynamic library for the actual runtime environment, or statically<br/>
compiled in for cases where the target architecture is known.<br/>
<br/>
Chapter 21 String<br/>
------------------<br/>
Headers: <string><br/>
C headers: <cctype> <cwctype> <cstring> <cwchar> (also in 27)<br/>
<cstdlib> (also in 18, 25, 26)<br/>
<br/>
We have "mostly-complete" char_traits<> implementations. Many of the<br/>
char_traits<char> operations might be optimized further using existing<br/>
proprietary language extensions.<br/>
<br/>
We have a "mostly-complete" basic_string<> implementation. The work<br/>
to manually instantiate char and wchar_t specializations in object<br/>
files to improve link-time behavior is extremely unsatisfactory,<br/>
literally tripling library-build time with no commensurate improvement<br/>
in static program link sizes. It must be redone. (Similar work is<br/>
needed for some components in chapters 22 and 27.)<br/>
<br/>
Other work needed for strings is MT-safety, as discussed under the<br/>
chapter 20 heading.<br/>
<br/>
The standard C type mbstate_t from <cwchar> and used in char_traits<><br/>
must be different in C++ than in C, because in C++ the default constructor<br/>
value mbstate_t() must be the "base" or "ground" sequence state.<br/>
(According to the likely resolution of a recently raised Core issue,<br/>
this may become unnecessary. However, there are other reasons to<br/>
use a state type not as limited as whatever the C library provides.)<br/>
If we might want to provide conversions from (e.g.) internally-<br/>
represented EUC-wide to externally-represented Unicode, or vice-<br/>
versa, the mbstate_t we choose will need to be more accommodating<br/>
than what might be provided by an underlying C library.<br/>
<br/>
There remain some basic_string template-member functions which do<br/>
not overload properly with their non-template brethren. The infamous<br/>
hack akin to what was done in vector<> is needed, to conform to<br/>
23.1.1 para 10. The CHECKLIST items for basic_string marked 'X',<br/>
or incomplete, are so marked for this reason.<br/>
<br/>
Replacing the string iterators, which currently are simple character<br/>
pointers, with class objects would greatly increase the safety of the<br/>
client interface, and also permit a "debug" mode in which range,<br/>
ownership, and validity are rigorously checked. The current use of<br/>
raw pointers as string iterators is evil. vector<> iterators need the<br/>
same treatment. Note that the current implementation freely mixes<br/>
pointers and iterators, and that must be fixed before safer iterators<br/>
can be introduced.<br/>
<br/>
Some of the functions in <cstring> are different from the C version.<br/>
generally overloaded on const and non-const argument pointers. For<br/>
example, in <cstring> strchr is overloaded. The functions isupper<br/>
etc. in <cctype> typically implemented as macros in C are functions<br/>
in C++, because they are overloaded with others of the same name<br/>
defined in <locale>.<br/>
<br/>
Many of the functions required in <cwctype> and <cwchar> cannot be<br/>
implemented using underlying C facilities on intended targets because<br/>
such facilities only partly exist.<br/>
<br/>
Chapter 22 Locale<br/>
------------------<br/>
Headers: <locale><br/>
C headers: <clocale><br/>
<br/>
We have a "mostly complete" class locale, with the exception of<br/>
code for constructing, and handling the names of, named locales.<br/>
The ways that locales are named (particularly when categories<br/>
(e.g. LC_TIME, LC_COLLATE) are different) varies among all target<br/>
environments. This code must be written in various versions and<br/>
chosen by configuration parameters.<br/>
<br/>
Members of many of the facets defined in <locale> are stubs. Generally,<br/>
there are two sets of facets: the base class facets (which are supposed<br/>
to implement the "C" locale) and the "byname" facets, which are supposed<br/>
to read files to determine their behavior. The base ctype<>, collate<>,<br/>
and numpunct<> facets are "mostly complete", except that the table of<br/>
bitmask values used for "is" operations, and corresponding mask values,<br/>
are still defined in libio and just included/linked. (We will need to<br/>
implement these tables independently, soon, but should take advantage<br/>
of libio where possible.) The num_put<>::put members for integer types<br/>
are "mostly complete".<br/>
<br/>
A complete list of what has and has not been implemented may be<br/>
found in CHECKLIST. However, note that the current definition of<br/>
codecvt<wchar_t,char,mbstate_t> is wrong. It should simply write<br/>
out the raw bytes representing the wide characters, rather than<br/>
trying to convert each to a corresponding single "char" value.<br/>
<br/>
Some of the facets are more important than others. Specifically,<br/>
the members of ctype<>, numpunct<>, num_put<>, and num_get<> facets<br/>
are used by other library facilities defined in <string>, <istream>,<br/>
and <ostream>, and the codecvt<> facet is used by basic_filebuf<><br/>
in <fstream>, so a conforming iostream implementation depends on<br/>
these.<br/>
<br/>
The "long long" type eventually must be supported, but code mentioning<br/>
it should be wrapped in #if guards to allow pedantic-mode compiling.<br/>
<br/>
Performance of num_put<> and num_get<> depend critically on<br/>
caching computed values in ios_base objects, and on extensions<br/>
to the interface with streambufs.<br/>
<br/>
Specifically: retrieving a copy of the locale object, extracting<br/>
the needed facets, and gathering data from them, for each call to<br/>
(e.g.) operator<< would be prohibitively slow. To cache format<br/>
data for use by num_put<> and num_get<> we have a _Format_cache<><br/>
object stored in the ios_base::pword() array. This is constructed<br/>
and initialized lazily, and is organized purely for utility. It<br/>
is discarded when a new locale with different facets is imbued.<br/>
<br/>
Using only the public interfaces of the iterator arguments to the<br/>
facet functions would limit performance by forbidding "vector-style"<br/>
character operations. The streambuf iterator optimizations are<br/>
described under chapter 24, but facets can also bypass the streambuf<br/>
iterators via explicit specializations and operate directly on the<br/>
streambufs, and use extended interfaces to get direct access to the<br/>
streambuf internal buffer arrays. These extensions are mentioned<br/>
under chapter 27. These optimizations are particularly important<br/>
for input parsing.<br/>
<br/>
Unused virtual members of locale facets can be omitted, as mentioned<br/>
above, by a smart linker.<br/>
<br/>
Chapter 23 Containers<br/>
----------------------<br/>
Headers: <deque> <list> <queue> <stack> <vector> <map> <set> <bitset><br/>
<br/>
All the components in chapter 23 are implemented in the SGI STL.<br/>
They are "mostly complete"; they include a large number of<br/>
nonconforming extensions which must be wrapped. Some of these<br/>
are used internally and must be renamed or duplicated.<br/>
<br/>
The SGI components are optimized for large-memory environments. For<br/>
embedded targets, different criteria might be more appropriate. Users<br/>
will want to be able to tune this behavior. We should provide<br/>
ways for users to compile the library with different memory usage<br/>
characteristics.<br/>
<br/>
A lot more work is needed on factoring out common code from different<br/>
specializations to reduce code size here and in chapter 25. The<br/>
easiest fix for this would be a compiler/ABI improvement that allows<br/>
the compiler to recognize when a specialization depends only on the<br/>
size (or other gross quality) of a template argument, and allow the<br/>
linker to share the code with similar specializations. In its<br/>
absence, many of the algorithms and containers can be partial-<br/>
specialized, at least for the case of pointers, but this only solves<br/>
a small part of the problem. Use of a type_traits-style template<br/>
allows a few more optimization opportunities, more if the compiler<br/>
can generate the specializations automatically.<br/>
<br/>
As an optimization, containers can specialize on the default allocator<br/>
and bypass it, or take advantage of details of its implementation<br/>
after it has been improved upon.<br/>
<br/>
Replacing the vector iterators, which currently are simple element<br/>
pointers, with class objects would greatly increase the safety of the<br/>
client interface, and also permit a "debug" mode in which range,<br/>
ownership, and validity are rigorously checked. The current use of<br/>
pointers for iterators is evil.<br/>
<br/>
As mentioned for chapter 24, the deque iterator is a good example of<br/>
an opportunity to implement a "staged" iterator that would benefit<br/>
from specializations of some algorithms.<br/>
<br/>
Chapter 24 Iterators<br/>
---------------------<br/>
Headers: <iterator><br/>
<br/>
Standard iterators are "mostly complete", with the exception of<br/>
the stream iterators, which are not yet templatized on the<br/>
stream type. Also, the base class template iterator<> appears<br/>
to be wrong, so everything derived from it must also be wrong,<br/>
currently.<br/>
<br/>
The streambuf iterators (currently located in stl/bits/std_iterator.h,<br/>
but should be under bits/) can be rewritten to take advantage of<br/>
friendship with the streambuf implementation.<br/>
<br/>
Matt Austern has identified opportunities where certain iterator<br/>
types, particularly including streambuf iterators and deque<br/>
iterators, have a "two-stage" quality, such that an intermediate<br/>
limit can be checked much more quickly than the true limit on<br/>
range operations. If identified with a member of iterator_traits,<br/>
algorithms may be specialized for this case. Of course the<br/>
iterators that have this quality can be identified by specializing<br/>
a traits class.<br/>
<br/>
Many of the algorithms must be specialized for the streambuf<br/>
iterators, to take advantage of block-mode operations, in order<br/>
to allow iostream/locale operations' performance not to suffer.<br/>
It may be that they could be treated as staged iterators and<br/>
take advantage of those optimizations.<br/>
<br/>
Chapter 25 Algorithms<br/>
----------------------<br/>
Headers: <algorithm><br/>
C headers: <cstdlib> (also in 18, 21, 26))<br/>
<br/>
The algorithms are "mostly complete". As mentioned above, they<br/>
are optimized for speed at the expense of code and data size.<br/>
<br/>
Specializations of many of the algorithms for non-STL types would<br/>
give performance improvements, but we must use great care not to<br/>
interfere with fragile template overloading semantics for the<br/>
standard interfaces. Conventionally the standard function template<br/>
interface is an inline which delegates to a non-standard function<br/>
which is then overloaded (this is already done in many places in<br/>
the library). Particularly appealing opportunities for the sake of<br/>
iostream performance are for copy and find applied to streambuf<br/>
iterators or (as noted elsewhere) for staged iterators, of which<br/>
the streambuf iterators are a good example.<br/>
<br/>
The bsearch and qsort functions cannot be overloaded properly as<br/>
required by the standard because gcc does not yet allow overloading<br/>
on the extern-"C"-ness of a function pointer.<br/>
<br/>
Chapter 26 Numerics<br/>
--------------------<br/>
Headers: <complex> <valarray> <numeric><br/>
C headers: <cmath>, <cstdlib> (also 18, 21, 25)<br/>
<br/>
Numeric components: Gabriel dos Reis's valarray, Drepper's complex,<br/>
and the few algorithms from the STL are "mostly done". Of course<br/>
optimization opportunities abound for the numerically literate. It<br/>
is not clear whether the valarray implementation really conforms<br/>
fully, in the assumptions it makes about aliasing (and lack thereof)<br/>
in its arguments.<br/>
<br/>
The C div() and ldiv() functions are interesting, because they are the<br/>
only case where a C library function returns a class object by value.<br/>
Since the C++ type div_t must be different from the underlying C type<br/>
(which is in the wrong namespace) the underlying functions div() and<br/>
ldiv() cannot be re-used efficiently. Fortunately they are trivial to<br/>
re-implement.<br/>
<br/>
Chapter 27 Iostreams<br/>
---------------------<br/>
Headers: <iosfwd> <streambuf> <ios> <ostream> <istream> <iostream><br/>
<iomanip> <sstream> <fstream><br/>
C headers: <cstdio> <cwchar> (also in 21)<br/>
<br/>
Iostream is currently in a very incomplete state. <iosfwd>, <iomanip>,<br/>
ios_base, and basic_ios<> are "mostly complete". basic_streambuf<> and<br/>
basic_ostream<> are well along, but basic_istream<> has had little work<br/>
done. The standard stream objects, <sstream> and <fstream> have been<br/>
started; basic_filebuf<> "write" functions have been implemented just<br/>
enough to do "hello, world".<br/>
<br/>
Most of the istream and ostream operators << and >> (with the exception<br/>
of the op<<(integer) ones) have not been changed to use locale primitives,<br/>
sentry objects, or char_traits members.<br/>
<br/>
All these templates should be manually instantiated for char and<br/>
wchar_t in a way that links only used members into user programs.<br/>
<br/>
Streambuf is fertile ground for optimization extensions. An extended<br/>
interface giving iterator access to its internal buffer would be very<br/>
useful for other library components.<br/>
<br/>
Iostream operations (primarily operators << and >>) can take advantage<br/>
of the case where user code has not specified a locale, and bypass locale<br/>
operations entirely. The current implementation of op<</num_put<>::put,<br/>
for the integer types, demonstrates how they can cache encoding details<br/>
from the locale on each operation. There is lots more room for<br/>
optimization in this area.<br/>
<br/>
The definition of the relationship between the standard streams<br/>
cout et al. and stdout et al. requires something like a "stdiobuf".<br/>
The SGI solution of using double-indirection to actually use a<br/>
stdio FILE object for buffering is unsatisfactory, because it<br/>
interferes with peephole loop optimizations.<br/>
<br/>
The <sstream> header work has begun. stringbuf can benefit from<br/>
friendship with basic_string<> and basic_string<>::_Rep to use<br/>
those objects directly as buffers, and avoid allocating and making<br/>
copies.<br/>
<br/>
The basic_filebuf<> template is a complex beast. It is specified to<br/>
use the locale facet codecvt<> to translate characters between native<br/>
files and the locale character encoding. In general this involves<br/>
two buffers, one of "char" representing the file and another of<br/>
"char_type", for the stream, with codecvt<> translating. The process<br/>
is complicated by the variable-length nature of the translation, and<br/>
the need to seek to corresponding places in the two representations.<br/>
For the case of basic_filebuf<char>, when no translation is needed,<br/>
a single buffer suffices. A specialized filebuf can be used to reduce<br/>
code space overhead when no locale has been imbued. Matt Austern's<br/>
work at SGI will be useful, perhaps directly as a source of code, or<br/>
at least as an example to draw on.<br/>
<br/>
Filebuf, almost uniquely (cf. operator new), depends heavily on<br/>
underlying environmental facilities. In current releases iostream<br/>
depends fairly heavily on libio constant definitions, but it should<br/>
be made independent. It also depends on operating system primitives<br/>
for file operations. There is immense room for optimizations using<br/>
(e.g.) mmap for reading. The shadow/ directory wraps, besides the<br/>
standard C headers, the libio.h and unistd.h headers, for use mainly<br/>
by filebuf. These wrappings have not been completed, though there<br/>
is scaffolding in place.<br/>
<br/>
The encapsulation of certain C header <cstdio> names presents an<br/>
interesting problem. It is possible to define an inline std::fprintf()<br/>
implemented in terms of the 'extern "C"' vfprintf(), but there is no<br/>
standard vfscanf() to use to implement std::fscanf(). It appears that<br/>
vfscanf but be re-implemented in C++ for targets where no vfscanf<br/>
extension has been defined. This is interesting in that it seems<br/>
to be the only significant case in the C library where this kind of<br/>
rewriting is necessary. (Of course Glibc provides the vfscanf()<br/>
extension.) (The functions related to exit() must be rewritten<br/>
for other reasons.)<br/>
<br/>
<br/>
Annex D<br/>
-------<br/>
Headers: <strstream><br/>
<br/>
Annex D defines many non-library features, and many minor<br/>
modifications to various headers, and a complete header.<br/>
It is "mostly done", except that the libstdc++-2 <strstream><br/>
header has not been adopted into the library, or checked to<br/>
verify that it matches the draft in those details that were<br/>
clarified by the committee. Certainly it must at least be<br/>
moved into the std namespace.<br/>
<br/>
We still need to wrap all the deprecated features in #if guards<br/>
so that pedantic compile modes can detect their use.<br/>
<br/>
Nonstandard Extensions<br/>
----------------------<br/>
Headers: <iostream.h> <strstream.h> <hash> <rbtree><br/>
<pthread_alloc> <stdiobuf> (etc.)<br/>
<br/>
User code has come to depend on a variety of nonstandard components<br/>
that we must not omit. Much of this code can be adopted from<br/>
libstdc++-v2 or from the SGI STL. This particularly includes<br/>
<iostream.h>, <strstream.h>, and various SGI extensions such<br/>
as <hash_map.h>. Many of these are already placed in the<br/>
subdirectories ext/ and backward/. (Note that it is better to<br/>
include them via "<backward/hash_map.h>" or "<ext/hash_map>" than<br/>
to search the subdirectory itself via a "-I" directive.<br/>
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